1. Field of the Invention
The present invention relates to a separator for a battery using an organic electrolytic solution. More particularly, the present invention is concerned with a separator which comprises a mixture of a polyethylene and a propylene polymer and is of a uniform three-dimensional porous structure having fine pores and which exhibits excellent chemical resistance, mechanical properties and ion permeability. The separator of the present invention can advantageously be used in batteries, especially lithium batteries, such as spiral type primary and secondary lithium batteries (in which rapid advances have recently been made) or in other types of primary and secondary batteries using an organic electrolytic solution, especially while enjoying excellent safety. The present invention is also concerned with a method for producing the separator.
2. Description of the Related Art
Various types of microporous films of olefin polymers are known. For example, Japanese Patent Application Laid-Open Specification No. 2-94356 discloses a microporous film of polyethylene, but the film has poor mechanical properties. Japanese Patent Application Laid-Open Specification No. 64-70538 discloses a microporous film of a composition comprising an ultrahigh molecular weight polyethylene and a polypropylene. However, this composition requires a high temperature for extrusion and, therefore, when it is attempted to form a film from the composition by the use of an organic liquid (as a plasticizer which is to be extracted after the film formation for rendering the film porous), decomposition of the organic liquid is likely to occur, leading to difficulties in film formation. Further, Japanese Patent Application Publication Specification No. 46-40119 discloses a microporous film of a polypropylene, but the film has drawbacks in that it lacks a uniform three-dimensional porous structure and has a porosity as low as from only 30 to 40%, leading to a poor ion permeability. Moreover, the films described in the above-mentioned three patent documents have a disadvantage particularly with respect to safety, when these films are used as separators for lithium batteries, particularly for spiral type primary and secondary lithium batteries, in which rapid advances have recently been made or for other types of primary and secondary batteries using an organic electrolytic solution.
The terminology "safety" used in the present invention is explained below. When a battery undergoes external short-circuiting, Joule heat is generated by the short-circuiting, thereby causing the temperature of the battery to be elevated, so that a microporous film used as the battery separator is deformed or melted. The deformation of the porous film causes the pore diameter of the porous film to be reduced, thereby increasing the electrical resistance (corresponding to anti air permeability). The melting of the porous film causes the pores thereof to disappear thereby rendening the film poreless (the temperature at which the anti air permeability of the porous film becomes 500 sec/100 cc per sheet or more is referred to as the "pore-eliminating temperature"). The lower the temperature at which the electrical resistance of the microporous film is increased, or the pore-eliminating temperature of the microporous film, the lower the temperature at which the flow of ions is inhibited, thus preventing the internal temperature of the battery from being further increased. Therefore, when the temperature at which the electrical resistance of the microporous film is increased, or the pore-eliminating temperature of the microporous film is low, the internal temperature of the battery does not reach the melting point of lithium or the flash point of an organic electrolytic solution, so that the occurrence of accidents, such as fire, is prevented, thus assuring safety. More importantly once the internal temperature of a battery has risen, even when the electrical resistance of the separator has become increased, the once elevated internal temperature is not likely to sharply drop. It is even possible for the internal temperature to continue to rise. In the battery separator in which the pores have been eliminated by melting, the melt viscosity of the separator film is lowered with a further increase in the temperature, and when the temperature has reached a certain point, the separator suffers breakage. The higher the temperature at which the separator breaks (which is referred to as "film-breaking temperature" and at which the anti air permeability becomes substantially zero), the longer the period of time during which the flow of ions is inhibited, thereby surely preventing the temperature from rising. Thus, it can be considered that the higher the film-breaking temperature, the higher the safety. The present inventor has found that a specific characteristic of a separator, which has conventionally been not recognized, namely, the difference between the film-breaking temperature and the pore-eliminating temperature, is a decisive factor for assuring safety. Specifically, a battery separator in which the difference between the above-mentioned two characteristic temperatures is large can be considered to be a battery separator having high safety.
Illustratively stated, a separator comprised of a polyethylene (an ultrahigh molecular weight polyethylene, a high molecular weight polyethylene, or a mixture of an ultrahigh molecular weight polyethylene and a high molecular weight polyethylene) alone has a low pore-eliminating temperature, but also has a low film-breaking temperature, so that the safety of such a separator cannot be considered to be high.
On the other hand, a separator comprised of a propylene polymer alone has a high pore-eliminating temperature, so that the safety is low.
Conventionally, an ultrahigh molecular weight polyethylene has excellent mechanical properties, but has poor molding properties because of its low flowability. When a mixture of an ultrahigh molecular weight polyethylene, inorganic particles and an organic liquid, is subjected to extrusion molding for obtaining a microporous film which is improved in mechanical properties, a pressure increase occurs in the molding, thus causing difficulties in molding. Further, a mixture of an ultrahigh molecular weight polyethylene, a high molecular weight polyethylene, inorganic particles and an organic liquid can be conceived. In the extrusion molding of the mixture, however, when the ratio of the ultrahigh molecular weight polyethylene to the other components is increased, an increase in the molding pressure occurs, thereby causing difficulties in the molding. Consequently, in this case, it is impossible to enhance the mechanical properties by increasing the ratio of the ultrahigh molecular weight polyethylene.